Plasmacytomas of NZB, BALB/c, and (BALB/c X NZB)F mice were compared to determine what genetically determined factors, including murine leukemia viruses, contribute to the unique susceptibility of these strains to plasmacytoma induction. Polyclonal B cell activation may be the major determining factor. A unique plasmacytoma of mice, closely resembling human Mott cell tumors, was detected during a study of BALB/c plasmacytomas. These cells produce, but do not secrete, IgA immunoglobulin.